This invention relates to a corrosion resistant lid used to close a semiconductor package and, in particular, to a lid having a low porosity multilayer coating.
In many semiconductor applications, the semiconductor device is required to be hermetically sealed within a "package" or housing which protects the device from the surrounding ambient and thus enhances its reliability. As explained in further detail in U.S. Pat. Nos. 3,340,602; 3,538,597; 3,874,549 and 3,946,190, the package typically includes a container having a cavity in which the device is securely seated. Electrical leads are passed out of the container and are connected to appropriate circuitry. The package is closed using a lid which is placed in registration over the cavity and sealed in place using a eutectic solder frame formed of an alloy made up of 80% gold and 20% tin.
The lid that is used throughout the industry is typically made from a Kovar stamping. Kovar is a well known tradename that identifies an alloy containing various amounts of cobalt, nickel and iron. Lids have heretofore been provided with a top coating of gold that is placed over an inner layer of nickel. The coating provides an excellent bonding surface for the solder frame and also provides a corrosion resistant shield for the Kovar substrate which, because it contains iron, is highly susceptible to rust damage. The nickel interface usually consists of between 50 and 150 microinches of low stress nickel while the top coat consists of about 50 microinches of pure gold. Although this dual combination exhibits good solderability, the lid nevertheless will rust when exposed to a corrosive atmosphere for any period of time. Corrosion in amounts of between 2-4% of the total surface area of the lid will generally occur within 24 to 96 hours when the coated lid is exposed to a salt containing atmosphere.
The accepted standards in the industry which governs the amount of corrosion allowable for high reliability packages is set out in the military specification Mil. Std. 883 C. This specification has been recently revised so that all lids now must remain corrosion free (zero corrosion) after being exposed to a salt containing atmosphere for at least 24 hours. Dual coated lids found in the prior use, i.e. those having a first coating of nickel and a top coating of gold, continually fail the corrosion test as set out in these specifications.
All electroplated metal coatings exhibit porosity to some extent and thus permit rust producing atmospheres to pass therethrough to the base metal. Methods have been tried with varying degrees of success to reduce the porosity of protective coatings and to increase the resistance of these lids to corrosion. Porosity is usually inversely proportional to the thickness of an electroplated metal and the pores that initially form in the coating material close gradually as more metal is deposited. As the coating thickens, the pores eventually close. Approximately 2000 microinches of nickel and about 100 microinches of gold are required, however, to completely close the pores on a dual coated lid. (See Harper, Charles A., Handbook of Materials and Processing for Electronics. McGraw Hill, 1970, p. 10-56). The consumption of this amount of metal is not only expensive but also requires an extraordinary amount of time to complete the plating process.
Pulse plating has also been tried with some limited success in an effort to close the pores in the coating materials. In this process, the current applied to the electroplating tank is pulsed on and off periodically by a square wave generator. The pulsing provides for increased ion mobility in the bath which, in turn, results in a smaller more densely packed crystal structure in the electroplated metals. This denser deposit is believed to fill the pores more rapidly and thus provide greater protection for a given coating thickness. Although the amount of corrosion may be reduced by this technique, pulse coating alone cannot provide economically feasible products capable of meeting the new standard within the industry.